<div>
<span>first span</span>
</div>
<div>
  <h5>Original content</h5>
  <p>
    inside paragraph
    <span>inside span</span>
    inside paragraph
  </p>
</div>
<div>
  <h5>Output</h5>
  <div id="output"></div>
</div>

<script>
var baseElement = document.querySelector("p");
document.getElementById("output").innerHTML =
  (baseElement.querySelector("div span").innerHTML);
</script>